A surround sound processor operates to enhance a two-channel stereophonic source signal so as to drive a multiplicity of loudspeakers arranged to surround the listener, in a manner to provide a high-definition soundfield directly comparable to discrete multitrack sources in perceived performance. An illusion of space may thus be created enabling the listener to experience the fullness, directional quality and aural dimension or "spaciousness" of the original sound environment. The foregoing so-called periphonic reproduction of sound can be distinguished from the operation of conventional soundfield processors which rely on digitally generated time delay of audio signals to simulate reverberation or "ambience" associated with live sound rooms. These conventional systems do not directionally localize sounds based on information from the original performance space and the resulting reverberation characteristics are noticeably artificial.
To accomplish this end, a surround sound processor typically comprises an input signal conditioning and matrix circuit, a control voltage generator circuit and a variable matrix circuit.
The input conditioning and matrix circuit usually provides for balance and level control of the input signals, generates normal and inverted polarity versions of the input signals, plus sum and difference signals derived therefrom, and in some cases generates phase-shifted versions thereof. Additionally, the signals may be filtered and split into multiple frequency ranges as needed by the remainder of the processing requirements.
The control voltage generator typically includes one or more band-pass filter circuits, directional detector circuits, and servologic circuits. The band-pass filter circuits weight the frequency response of the audio signals applied to the directional detector circuits so that these circuits respond similarly to the human ear. The directional detectors measure the correlations between the signals, which represent sounds encoded at different directions in the stereophonic sound stage, generating voltages corresponding to the directional location of the predominant sound. The servologic circuits use these signals to develop control voltage signals for varying the gains of voltage-controlled amplifiers in the variable matrix circuit in accordance with the original sound direction and the direction in which it is intended to reproduce the sound in the surrounding loudspeakers.
The variable matrix circuit includes a number of voltage-controlled amplifiers and a separation matrix. the voltage-controlled amplifiers amplify the audio signals from the input conditioning and matrix circuit with variable gain, which is controlled by the control voltage signals, and the resulting audio signals are applied to the separation matrix circuit where they are used to selectively cancel unwanted crosstalk in the different loudspeaker output signals. The separation matrix combines the outputs of the input conditioning and matrix with those of the voltage-controlled amplifiers in several different combinations, each resulting in a loudspeaker output signal intended for driving (through suitable power amplifiers) a loudspeaker positioned at a particular direction relative to the listener. In each of these loudspeaker output signals, certain unwanted crosstalk components may be dynamically eliminated by the action of the direction detector circuits, servologic circuits, voltage-controlled amplifiers, and separation matrix circuit.
In Fosgate's co-pending patent application Ser. No. 07/533,091, entitled "Surround Processor" a servologic circuit is disclosed. This circuit employs a smoothing filter with a variable time constant to smooth the directional information signal from each directional detector circuit, to produce a control voltage signal therefrom, each time constant being varied inversely with the absolute magnitude of the difference between the directional information signal and the corresponding control voltage signal. This is done by means of a width-modulated pulse train controlling a switch which selects between two resistors of different values, effectively varying the resistance according to the duty ratio of the width-modulated pulse train and changing the effective time constant with an associated capacitor in a continuous manner. Thus the modulating elements are placed within a negative feedback loop, providing a servo control system, whence is derived the term "servologic circuit."
In his co-pending application Ser. No. 07/789,530, Fosgate discloses the addition to this basic circuit of a one-shot circuit and threshold detector. The one-shot is designed to force the duty ratio of the width modulated pulse train to unity for a specific short period of time, causing all of the variable time constants to be reduced to their minimum values and thereby allowing the smoothed control voltage signals to catch up to their corresponding directional information signals rapidly, whenever the differences between them increase beyond the threshold level which triggers the one-shot. A one-shot was also disclosed in Fosgate's U.S. Pat. No. 4,932,059, this being triggered from a special attack detector circuit for sensing the rapid onset of new signals.
In the present invention, directed to circuitry suitable for inclusion in an integrated circuit for performing the servologic circuit functions, an alternative method of varying the time constants by means of a multiplier circuit is disclosed, and a novel circuit combining the functions of a threshold detector and one-shot is described. Furthermore, it is shown that the servologic circuit function can be effected by means of a nonlinear resistive network forming a variable time constant with a fixed capacitor.
The dynamic characteristics of the control voltage generator have a significant impact on the perceived quality of the spatial impression generated by a surround processor, and as the control voltage generator circuit is improved, the processor exhibits greater freedom from unwanted audible artifacts and improved dynamic separation.